1. Field of the Invention
The present invention relates to a reflective photosensor for detecting the position of a target object such as a sheet of paper and an image forming device incorporating such a reflective photosensor.
2. Description of the Related Art
Japanese Patent Application Publication No. 2010-235313 discloses an image forming device partially shown in FIG. 11, for example. It includes a photosensor 832 with a filler 832a and a transmissive photosensor 832b to detect the position of a paper sheet P carried by a feed roller 802 and else. The filler 832a is arranged on the carrier path of the paper sheet P and hit and swung by the paper sheet P. The transmissive photosensor 832b includes a light emitter and a light receiver facing each other so that the swung filler 832a shields light traveling from the light emitter to the light receiver. The filler photosensor 832 is configured to detect the position of the carried paper sheet P from shielded or unshielded light.
Such a filler photosensor 832 faces problems that since the speed at which the paper sheet P is carried is restricted by the swing speed of the filler 832a, it is difficult to increase it, and that the paper sheet P may hit the filler 832a and the tip end thereof may be damaged.
In view of the above problems, Japanese Patent Application Publication No. 5-55635 discloses an image forming device with a reflective photosensor in replace of a filler photosensor as shown in FIG. 12, for example. A reflective photosensor 901 includes a transparent circuit board 902 and a light emitter 904 with a light emitting element 904a and a light receiver 905 with a light receiving element 905a on one surface 902a of the circuit board 902. Both the light emitting element 905a and light receiving element 905b face the one surface 902a. Thus, the light emitter 904 emits light K to the paper sheet P through the circuit board 902 and the light receiver 905 receives light components reflected by the paper sheet P through the circuit board 902.
The light K emitted from the light emitter 904 is received by the light receiver 905 when the paper sheet P is in front of the reflective photosensor 901 and not received when the sheet is not in front of the reflective photosensor 901. Accordingly, this reflective photosensor 901 can detect the position of the carried paper sheet P in non-contact manner from the receipt or non-receipt of the light K by the light receiver 905. Without a moving element as the filler, it is possible to carry the paper sheet P at a higher speed and prevent it from being damaged.
However, there is a problem with the reflective photosensor 901 in low detection accuracy since the light receiving element 905a receives, off the axis, the light components K in a low amount diffusely reflected by the paper sheet P. This is due to a parallel relation of the axes of the light emitting element 904a and light receiving element 905a both disposed to face the one surface 902a. A lens element or a cover can be additionally provided to change the traveling direction of light K and enable the light receiving element 905a to receive light specularly reflected by the paper sheet P. However, this leads to increasing the outer form of the reflective photosensor.